So you've sexted the wrong person by mistake - what do you do next? There's no way to "undo" a sext, unfortunately, so all you can do is manage the situation you've created to keep awkwardness to a minimum and get things back on track with the individual you sexted. So here's a step-by-step guide to fixing that situation in the least uncomfortable way possible!
You can't just send someone the first BIT of a sext and then drop off or say "oops" - doing that reveals that you made a mistake, were clumsy and unattentive with your private communications, and could potentially make the person feel bad (what if they had a crush on you and were PUMPED to get a sext from you, only for you to take it back after getting their hopes up and say "just kidding!"). So, there's only one option: COMMIT.
Yes, I'm saying you continue with the sexting until completion. Act like you're did this on purpose and with them in mind, regardless of who it was. You may have to recast yourself as something of a very open-minded sex positive individual who has ongoing sexts with PRETTY MUCH EVERYONE (because sexuality is a natural part of humanity and there is no wisdom in limiting your sext experiences to just your partner), but then you won't have to worry about awkwardness of admitting you accidentally sent a sext to your dad instead of your girlfriend.
If you're going to sell this person on the idea that you're a wildly sex positive person who doesn't believe in the traditional norms and boundaries of society (e.g. "not sexting your dad"), you're going to need to back it up with actual proof that you're living that lifestyle and that mindset. What if they asked your other friends, relatives, and acquaintances if THEY'D ever received and out-of-the-blue sext....and they said "Of course not." Boom. Game over. Your house of cards has come crashing down.
So you know what you have to do - begin a series of unprompted sexts with every person in your phonebook. Aunts, old college friends, dog walkers - it doesn't matter. After all - it's not ACTUAL sex, so what's the harm?
Oh, so you think you can sell the idea that you're just this sex positive person BUT ONLY FOR TEXTS? That person doesn't exist - if you're living the sex positive lifestyle, you can't pretend like it only extends to your phone. You've got to open yourself up to as many new sexual experiences as you can imagine - from anonymous sex to group sex to kinkplay to BDSM to polyamory and everything in-between. Free yourself from the chains of monogamy and tradition - these are manmade concepts, not aligning with nature. Nature wants us to feel good - nature wants us to live lives of pleasure. And pleasure knows no boundaries. Remove all of your prudish instincts and dive headfirst into a nonstop bacchanal of orgies, lovemaking, and a maelstrom of sexual adventures you never thought possible.
Your life of sensual indulgence and hedonism has left a void within you - by seeking pleasures of the flesh, you've lost your sense of self and sense of stability. The sex is plentiful, but the connections simply aren't there. You would feel emotionally-drained if you felt as though you were still attached to emotions whatsoever. While your life of rules and boundaries seem quaint in your newly enlightened form, you find yourself longing for the simple joys of stability and support you once knew. But can you ever go back? Even if you wanted - could you forget the wisdom you have gained?
...or was it wisdom at all? Is it truly wisdom if it leaves you feeling more lost than before? Is ignorance truly bliss? Is the truest hell of all merely self-awareness?!
You must go back to the beginning - to the moment where everything went wrong... the original sin. The sext to your dad. The only option is to play it - and the last several years of your life - as a joke.
They'll probably be chill about it and laugh it off too. After all, it happens to everyone, right?